1. Field of the Invention
The present invention relates to image forming apparatus that carry out switchback transport for switching transport directions of recording papers.
2. Description of the Related Art
In this type of image forming apparatus, the transport directions of the recording papers are switched to selectively carry out printing on only the front surface (front side) of the recording papers and printing on both sides of the recording papers. When printing on only the front surface of the recording papers, the recording paper is immediately discharged after printing has been completed on the front surface of the recording paper. And when printing on both sides of the recording papers, transport of the recording paper is temporarily stopped after printing has been completed on the front surface of the recording paper, then the recording paper is transported in the reverse direction through a diversion path (switchback transport), and after the front and back of the recording paper have been turned over, printing is carried out on the back surface of the recording paper and the recording paper is discharged.
For example, a conventional electrophotographic image forming apparatus such as that shown in FIG. 5 is provided with a forward direction path 101 in which a recording paper P is transported in a forward direction, a diversion path 102 in which the recording paper P is transported in a reverse direction, pairs of transport rollers 103 arranged in multiple locations on the forward direction path 101 and the diversion path 102, a photosensitive drum 104 and a transfer roller 105, a heating roller 106 and a pressure roller 107, a pair of discharge rollers 108 arranged at the terminal end of the forward direction path 101, and a discharge tray 109. In this image forming apparatus, a toner image is formed on the surface of the photosensitive drum 104 while the recording paper P is transported along the forward direction path 101, then the recording paper P is sandwiched in a nip region between the photosensitive drum 104 and the transfer roller 105 to transfer the toner image on the surface of the photosensitive drum 104 onto the recording paper P, then the recording paper P is subjected to heat and pressure by being sandwiched in a nip region between the heating roller 106 and the pressure roller 107, thereby fixing the toner image onto the recording paper P. When performing printing on only the front surface of the recording paper P, the recording paper P is transported along the forward direction path 101 such that a toner image is recorded onto the front surface, then the recording paper P is discharged to the discharge tray 109 by the discharge rollers 108.
Furthermore, when printing on both surfaces of the recording paper P, after printing has been completed on the front surface of the recording paper P, rotation is temporarily stopped of the discharge rollers 108, which are sandwiching the recording paper P midway during discharge of the recording paper to the discharge tray 109, then the discharge rollers 108 are rotated in reverse to transport the recording paper P in the reverse direction and feed it to the diversion path 102 (switchback transport). This recording paper P is transported through the diversion path 102 and returned to the forward direction path 101 after its front and back are turned over, then it is again sandwiched in the nip region between the photosensitive drum 104 and the transfer roller 105, a toner image is recorded on its back surface, then it passes through the nip region between the heating roller 106 and the pressure roller 107, and is discharged to the discharge tray 109 by the discharge rollers 108.
Furthermore, in JP 2007-99480A, ribs that contact the recording papers in the forward direction path 101 and the diversion path 102 shown in FIG. 5 are formed so that the stress applied to the recording papers does not concentrate in a single location, thereby preventing noise and corner bending of the recording papers caused by contact between the recording papers and the ribs.
However, in conventional image forming apparatuses, when the rotation of the discharge rollers 108 is temporarily stopped to perform switchback transport, a phenomenon is produced that the leading edge (front edge) of the recording paper P being sandwiched between the discharge rollers 108 droops down on the discharge tray 109 side as shown in FIG. 6, and accompanying this the trailing edge of the recording paper P upwardly rises on the side opposite to the discharge tray 109 (the side of the forward direction path 101 and the diversion path 102). This phenomenon tends to occur particularly when the width of the nip region between the discharge rollers 108 is narrow, or when the discharge rollers 108 are provided in a divided manner with a space between the discharge rollers 108 in the lengthwise directions of the discharge rollers 108.
And when the recording paper P in this state is transported in the reverse direction, the raised trailing edge of the recording paper tends to catch on the ribs or the like in the forward direction path 101 and the diversion path 102, thereby producing noise or corner bending (dog ears) of the recording paper.
In JP 2007-99480A, although innovations are implemented in the formation of the ribs that contact the recording papers in the forward direction path 101 and the diversion path 102, these innovations do not extend to preventing the trailing edge of the recording papers from contacting or catching on the ribs or the like of the forward direction path 101 and the diversion path 102 during switchback transport, and it has not been possible to prevent the noise and corner bending of recording papers that accompanies switchback transport.